using System;

using System.Text;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(PlayPass("I LOVE YOU!!!", 1)); // "!!!vPz fWpM J"

        Console.WriteLine(PlayPass("MY GRANMA CAME FROM NY ON THE 23RD OF APRIL 2015", 2)); // "4897 NkTrC Hq fT67 GjV Pq aP OqTh gOcE CoPcTi aO"

    }

​

    /// <summary>

    /// #1. Shift each letter by a given number but the transformed letter must be a letter (circular shift),

    /// #2. Replace each digit by its complement to 9,

    /// #3. Keep such as non alphabetic and non digit characters,

    /// #4. Downcase each letter in odd position, upcase each letter in even position (the first character is in position 0),

    /// #5. Reverse the whole result.

    /// </summary>

    /// <param name="s">Passphrase in raw form</param>

    /// <param name="n">Number of shifts</param>

    /// <returns>Encoded Passphrase</returns>

    public static string PlayPass(string s, int n) 

    {       

        // #1

        s = CaesarCipher(s, n);     

​

        var sb = new StringBuilder();

        for(int i = 0; i < s.Length; i++)

        {

            char c = s[i];

            // #4.

            if(i % 2 != 0) sb.Append(Char.ToLower(s[i]));

            else sb.Append(Char.ToUpper(s[i]));

        }

        return sb.ToString();   

    }

​

    static string CaesarCipher(string value, int shift)

    {

        char[] buffer = value.ToCharArray();

        for (int i = 0; i < buffer.Length; i++)

        {

            char letter = buffer[i];

            letter = (char)(letter + shift);

            if (letter > 'z')

            {

                letter = (char)(letter - 26);

            }

            else if (letter < 'a')

            {

                letter = (char)(letter + 26);

            }

            buffer[i] = letter;

        }

        return new string(buffer);

    }

}